E. bieneusi is a common infection in those with AIDS causing diarrhea, malabsorption and wasting in the terminal stages. Inspite of its common occurance little is known regarding the basic biology, epidemiology and host immunity of this parasite. The applicants have been able to transmit and establish a persistant infection in SIV infected macaques using a parasite obtained from a human AIDS infection. Spontaneous infection with this parasite also occurs in macaques and is associated with hepatobiliary and intestinal disease. The applicant's hypothesis is that this parasite in macaques produces minimal clinical signs but establishes persistent diarrhea when the parasite is redistributed to selected areas of the hepatobiliary tree. Exacerbation of previously inapparent infection or acquisition of new infection during immunosuppression may result in expanded tissue distribution, dysfunction of the alimentary tract and progressive disease. In this proposal, the applicants propose to: 1) inoculate immunosuppressed monkeys with the parasite and determine distribution over time and its relationship to progressive suppression and functional changes, 2) to examine biological differences between rhesus amd human-derived microsporidia through evaluation of genetic markers and 3) develop methods for preparation of spores, production of antibody and quantification of parasites in tissue.